I found out that some don't have the /bin/false entry but I still can't login into them. An example -> proxy:x:13:13:proxy:/bin:/bin/sh
– PithikosMay 6 '14 at 19:38

1

That has to do with a passwordless login - SSH rejects blank passwords (the :x: part of the line). You can su - proxy for example, but there's no need to.
– Nathan CMay 6 '14 at 23:34

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:x: isn't a blank password, it means there is no password that will work. That's the field for the password hash and nothing will hash to just the letter x, so no matter what you enter as a password it won't work. Actually, that was true before /etc/shadow; that field in /etc/passwd isn't used any more, but :x: may still indicate that it's not possible to log in.
– Randy OrrisonMay 12 '14 at 19:36

Thats wrong. Password: An x character indicates that encrypted password is stored in /etc/shadow file. Please note that you need to use the passwd command to computes the hash of a password typed at the CLI or to store/update the hash of the password in /etc/shadow file. source: cyberciti.biz/faq/understanding-etcpasswd-file-format
– trietendSep 25 '18 at 10:25

These accounts are used to run services in the background. Your linux system will have a range of application doing a range of tasks in the background, as you correctly identified SQL is one such service. In order for these services to carry out activities it must have a user attached.

In order to preserve the security of your system these tasks cannot be carried out as root and instead are assigned accounts with do not have shell or login access as identified by /bin/false or /sbin/nologin. This also allows permissions to be assigned only to the files used by each application.

These users are not interactive users in the traditional sense, but users that run services on your box. as such you cannot easily log in as those users, nor should you. The accounts are either password-less (login disabled) or have a randomly generated password. Passwordless accounts can be invoked by root (usually at boot) using su to run the actual service.